Emulating std::bind in C

Question

I\'m using std::bind to provide a callback while abstracting some logic by binding some parameters first. i.e.

void start() {

    int secret_id = 43534;

           


        

Answer 1

As 6502 explained, it is not possible to do this in portable C without some kind of context argument being passed to the callback, even if it doesn't name secret_id directly. However, there are libraries such as Bruno Haible's trampoline that enable creation of C functions with additional information (closures) through non-portable means. These libraries do their magic by invoking assembly or compiler extensions, but they are ported to many popular platforms; if they support architectures you care about, they work fine.

Taken from the web, here is an example of code that trampoline enables is this higher-order function that takes parameters a, b, and c (analogous to your secret_id, and returns a function of exactly one parameter x that calculates a*x^2 + b*x + c:

#include 

static struct quadratic_saved_args {
    double a;
    double b;
    double c;
} *quadratic_saved_args;

static double quadratic_helper(double x) {
    double a, b, c;
    a = quadratic_saved_args->a;
    b = quadratic_saved_args->b;
    c = quadratic_saved_args->c;
    return a*x*x + b*x + c;
}

double (*quadratic(double a, double b, double c))(double) {
    struct quadratic_saved_args *args;
    args = malloc(sizeof(*args));
    args->a = a;
    args->b = b;
    args->c = c;
    return alloc_trampoline(quadratic_helper, &quadratic_saved_args, args);
}

int main() {
    double (*f)(double);
    f = quadratic(1, -79, 1601);
    printf("%g\n", f(42));
    free(trampoline_data(f));
    free_trampoline(f);
    return 0;
}